Ultraviolet light screening compositions

ABSTRACT

A UV screening composition comprising particles which are capable of absorbing UV light so that electrons and positively charged holes are formed within the particles, characterised in that the particles are adapted to minimise migration to the surface of the particles of the electrons and/or the positively charged holes when said particles are exposed to UV light in an aqueous environment.

BACKGROUND OF THE INVENTION

The present invention relates to UV screening compositions, methods fortheir preparation and their use. The invention in particular relates to,for example, compositions comprising particulate oxides, theirpreparation and their use as, for example, paints, plastics, coatings,pigments, dyes and compositions for topical application, in particular,for example, sunscreens.

The effects associated with exposure to sunlight are well known. Forexample, painted surfaces may become discoloured and exposure of skin toUVA and UVB light may result in, for example, sunburn, premature ageingand skin cancer.

Commercial sunscreens generally contain components which are able toreflect and/or absorb UV light. These components include, for example,inorganic oxides such as zinc oxide and titanium dioxide.

Titanium dioxide in sunscreens is generally formulated as “micronised”or “ultrafine” (20-50 nm) particles (so-called microreflectors) becausethey scatter light according to Rayleigh's Law, whereby the intensity ofscattered light is inversely proportional to the fourth power of thewavelength. Consequently, they scatter UVB light (with a wavelength offrom 290 to 320 nm) and UVA light (with a wavelength of from 320 to 400nm) more than the longer, visible wavelengths, preventing sunburn whilstremaining invisible on the skin.

However, titanium dioxide also absorbs UV light efficiently, catalysingthe formation of superoxide and hydroxyl radicals which may initiateoxidations. The crystalline forms of TiO₂, anatase and rutile, aresemiconductors with band gap energies of about 3.23 and 3.06 eVrespectively, corresponding to light of about 385 nm and 400 nm (1 eVcorresponds to 8066 cm⁻¹).

An incident photon is absorbed by titanium dioxide if its energy isgreater than the semiconductor band gap Eg shown in FIG. 1. As a resultan electron from the valence band (vb) is promoted into the conductionband (cb) (transition [1]). If the energy of the incident photon is lessthan Eg it will not be absorbed as this would require that the electronbe promoted to within the band gap and this energy state is forbidden.Once promoted, the electron relaxes to the bottom of the conduction band(transition [2]) with the excess energy being emitted as heat to thecrystal lattice.

When the electron is promoted it leaves behind a hole which acts as apositive particle in the valence band. Both the electron and the holeare then free to migrate around the titanium dioxide particle. Theelectron and hole may recombine emitting a photon of energy equal to theband gap energy. However, the lifetime of the electron/hole pair isquite long due to the specific nature of the electronic band structure.Thus there is sufficient time (ca. 10 ⁻¹¹s) for the electron and hole tomigrate to the surface and react with absorbed species.

In aqueous environments, the electrons react with oxygen, and the holeswith hydroxyl ions or water, forming superoxide and hydroxyl radicals:TiO₂ +hu →TiO₂(e ⁻ /h ⁺)→e ⁻(cb)+h ⁺(vb)e ⁻(cb)+O₂→O₂ ^(.−)→HO^(.) ₂h ⁺(vb)+OH^(−→) ^(.)OH

This has been studied extensively in connection with total oxidation ofenvironmental pollutants, especially with anatase, the more active form[A. Sclafani et al., J. Phys. Chem., (1996), 100, 13655-13661].

It has been proposed that such photo-oxidations may explain the abilityof illuminated titanium dioxide to attack biological molecules.Sunscreen titanium dioxide particles are often coated with compoundssuch as alumina, silica and zirconia which form hydrated oxides whichcan capture hydroxyl radicals and may therefore reduce surfacereactions. However, some TiO₂/Al₂O₃ and TiO₂/SiO₂ preparations exhibitenhanced activity [C. Anderson et al., J. Phys. Chem., (1997), 101,2611-2616].

As titanium dioxide may enter human cells, the ability of illuminatedtitanium dioxide to cause DNA damage has also recently been a matter ofinvestigation. It has been shown that particulate titanium dioxide asextracted from sunscreens and pure zinc oxide will, when exposed toillumination by a solar simulator, give rise to DNA damage both in vitroand in human cells [R. Dunford et al, FEBS Lett., (1997), 418, 87-90].

The present invention provides UV screening compositions which addressthe problems described above and are less liable to produce DNA damageon illumination than conventional sunscreen compositions.

SUMMARY OF THE INVENTION

The present invention accordingly provides UV screening compositionscomprising particles which are capable of absorbing UV light, especiallyUV light having a wavelength below 390 nm, so that electrons andpositively charged holes are formed within the particles, characterisedin that the particles are adapted to minimise migration to the surfaceof the particles of the electrons and/or the positively charged holeswhen said particles are exposed to UV light in an aqueous environment.It is believed that under these circumstances the production of hydroxylradicals is substantially reduced. Thus the production of hydroxylradicals may be substantially prevented.

The minimisation of migration to the surface of the particles of theelectrons and/or the positively charged holes may be tested by, forexample, looking for a reduction in the number of strand breaksinflicted on DNA by light in the presence of particles or UV screeningcompositions according to the present invention, as compared with thenumber of strand breaks observed in DNA on treatment with particles usedin conventional sunscreen compositions and light, or light alone.

The compositions according to the present invention may find applicationas paints, plastics, coatings, pigments, dyes and are particularlyfavoured for use in compositions for topical application, especially,for example, sunscreens.

The average primary particle size of the particles is generally fromabout 1 to 200 nm, for example about 50 to 150 nm, preferably from about1 to 100 nm, more preferably from about 1 to 50 nm and most preferablyfrom about 20 to 50 nm. For example, in sunscreens the particle size ispreferably chosen to avoid colouration of the final product. For thispurpose particles of about 50 nm or less may be preferred especially,for example, particles of about 3 to 20 nm, preferably about 3 to 10 nm,more preferably about 3 to 5 nm.

Where particles are substantially spherical then particle size will betaken to represent the diameter. However, the invention also encompassesparticles which are non-spherical and in such cases the particle sizerefers to the largest dimension.

In a first embodiment the present invention provides UV screeningcompositions comprising particles which contain luminescence trap sitesand/or killer sites. By luminescence trap sites and killer sites will beunderstood foreign ions designed to trap the electrons and positivelycharged holes and therefore inhibit migration.

These particles may be, for example, reduced zinc oxide particles,especially reduced zinc oxide particles of from about 100 to 200 nm insize or smaller, for example, from about 20 to 50 nm.

Such reduced zinc oxide particles may be readily obtained by heatingzinc oxide particles which absorb UV light, especially UV light having awavelength below 390 nm, and reemit in the UV in a reducing atmosphereto obtain reduced zinc oxide particles which absorb UV light, especiallyUV light having a wavelength below 390 nm, and reemit in the green,preferably at about 500 nm. It will be understood that the reduced zincoxide particles will contain reduced zinc oxide consistent withminimising migration to the surface of the particles of electrons and/orpositively charged holes such that when said particles are exposed to UVlight in an aqueous environment the production of hydroxyl radicals issubstantially reduced as discussed above.

The zinc oxide is preferably heated in an atmosphere of about 10%hydrogen and about 90nitrogen by volume, e.g. at about 800 ° C. and forabout 20 minutes.

It is believed that the reduced zinc oxide particles possess an excessof Zn²⁺ions within the absorbing core. These are localised states and assuch may exist within the band gap. Transitions [1] and [2] may occur asshown in FIG. 1. However, the electron and hole may then relax to theexcess Zn²⁺states (transition [3]) as shown in FIG. 2. Thus theelectrons and holes may be trapped so that they cannot migrate to thesurface of the particles and react with absorbed species. The electronsand holes may then recombine at the Zn²⁺states (transition [4])accompanied by the release of a photon with an energy equivalent to thedifference in the energy levels.

Alternatively, particles of the present invention may comprise a hostlattice incorporating a second component to provide luminescence trapsites and/or killer sites. The host lattice may be preferably selectedfrom oxides, especially, for example, TiO₂ and ZnO, or for example,phosphates, titanates, silicates, aluminates, oxysilicates, tungstatesand molybdenates. The second component may, for example, be selectedaccording to criteria such as ionic size. Second components suitable for5 use according to the present invention may, for example, be selectedfrom nickel, iron, chromium, copper, tin, aluminium, lead, silver,zirconium, manganese, zinc, cobalt and gallium ions. Preferably thesecond component is selected from iron, chromium, manganese and galliumions in the 3+state. Preferred particles according to the presentinvention comprise a titanium dioxide host lattice doped with manganeseions in the 3+state.

The optimum amount of the second component in the host lattice may bedetermined by routine experimentation. It will be appreciated that theamount of the second component may depend on the use of the UV screeningcomposition. For example, in UV screening compositions for topicalapplication, it may be desirable for the amount of the second componentin the host lattice to be low so that the particles are not coloured. Inthe case of titanium dioxide doped with manganese ions in the 3+state,0.5% manganese in the titanium dioxide host lattice has been shown to beeffective in reducing the rate of DNA damage inflicted by simulatedsunlight. However, amounts as low as 0.1% or less, for example 0.05%, oras high as 1% or above, for example 5% or 10%, may also be used.

The dopant ions may be incorporated into the host lattice by a bakingtechnique typically at from 600° C. to 1000° C. Thus, for example, theseparticles may be obtained in a known manner by combining a host latticewith a second component to provide luminescence trap sites and/or killersites.

It is envisaged that the mechanism of de-excitation for these particlesis as described above for the reduced zinc particles.

In a further embodiment the present invention provides UV screeningcompositions comprising particles which comprise a population of coatednanoparticles of a metal oxide. The metal oxide is preferably titaniumdioxide. The coating is typically a wide band gap material and ispreferably a surfactant selected from trioctylphosphine oxide [TOPO] andsodium hexametaphosphate [(NaPO₃)₆].

The nanoparticles are generally from 1 to 10 nm in size and possiblyfrom 1 to 5 nm in size.

It has been found that the nanoparticles may be obtained by dissolving atitanium salt, preferably titanium (IV) chloride, in an alcohol. Thealcohol is 5 generally selected from methanol, ethanol, propanol andbutanol. Preferably the alcohol is methanol or propanol. The dehydratingproperties of the alcohol may help to inhibit formation of the hydroxidephase. A surfactant is added to bind to the titanium ions and form asurface layer. Typically the ratio of titanium ions to surfactant is1:1.

The pH of the solution is then monitored while a solution of sodiumhydroxide in alcohol, preferably a 1M solution in methanol, is addeddropwise until the oxide nanoparticles precipitate. The alcohol isevaporated so that the oxide particles flocculate. The particles may bewashed to remove excess surfactant and the remaining alcohol is thenevaporated to leave the titanium dioxide nanoparticles as a powder.Preparation of the nanoparticles is generally carried out in an inertatmosphere, preferably an atmosphere of nitrogen or argon. A populationof the nanoparticles may then be combined to form the larger particlesof the present invention.

On absorption of UV light the electrons and holes produced may beconfined to a specific nanoparticle within the particles of the presentinvention thus minimising migration of the electrons and/or the holes tothe surface of the particles. The nanoparticles may also be beneficialin that the rate of recombination of the electrons and holes may beincreased. The electrons and holes may recombine with the emission of aphoton of energy equal to the band gap as shown in FIG. 3.

The particles of the present invention may have an inorganic or organiccoating. For example, the particles may be coated with oxides ofelements such as aluminium, zirconium or silicon. The particles of metaloxide may also be coated with one or more organic materials such aspolyols, amines, alkanolamines, polymeric organic silicon compounds, forexample, RSi[OSi(Me)₂xOR¹]₃ where R is C¹⁻C₁₀ alkyl, R¹ is methyl orethyl and x is an integer of from 4 to 12, hydrophilic polymers such aspolyacrylamide, polyacrylic acid, carboxymethyl cellulose and xanthangum or surfactants such as, for example, TOPO.

As indicated above, compositions of the invention may be used in a widerange of applications where UV screening is desired, but areparticularly preferred for topical application. The compositions fortopical application may be, for example, cosmetic compositions,compositions for protecting the hair, or preferably sunscreens.Compositions of the present invention may be employed as anyconventional formulation providing protection from UV light.

In compositions for topical application, the metal oxides are preferablypresent at a concentration of about 0.5 to 10 % by weight, preferablyabout 3 to 8 % by weight and more preferably about 5 to 7% by weight.Such compositions may comprise one or more of the compositions of thepresent invention.

The compositions for topical application may be in the form of lotions,e.g. thickened lotions, gels, vesicular dispersions, creams, milks,powders, solid sticks and may be optionally packaged as aerosols andprovided in the form of foams or sprays.

The compositions may contain, for example, fatty substances, organicsolvents, silicones, thickeners, demulcents, other UVA, UVB orbroad-band sunscreen agents, antifoaming agents, moisturizing agents,perfumes, preservatives, surface-active agents, fillers, sequesterants,anionic, cationic, nonionic or amphoteric polymers or mixtures thereof,propellants, alkalizing or acidifying agents, colorants and metal oxidepigments with a particle size of from 100 nm to 20000 nm such as ironoxides.

The organic solvents may be selected from lower alcohols and polyolssuch as ethanol, isopropanol, propylene glycol, glycerin and sorbitol.

The fatty substances may consist of an oil or wax or mixture thereof,fatty acids, fatty acid esters, fatty alcohols, vaseline, paraffin,lanolin, hydrogenated lanolin or acetylated lanolin.

The oils may be selected from animal, vegetable, mineral or syntheticoils and especially hydrogenated palm oil, hydrogenated castor oil,vaseline oil, paraffin oil. Purcellin oil, silicone oil and isoparaffin.

The waxes may be selected from animal, fossil, vegetable, mineral orsynthetic waxes. Such waxes include beeswax, Carnauba, Candelilla, sugarcane or Japan waxes, ozokerites, Montan wax, microcrystalline waxes,paraffins or silicone waxes and resins.

The fatty acid esters are, for example, isopropyl myristate, isopropyladipate, isopropyl palmitate, octyl palmitate, C¹²⁻C₁₅ fatty alcoholbenzoates (“FINSOLV TN” from FINETEX), oxypropylenated myristic alcoholcontaining 3 moles of propylene oxide (“WITCONOL APM” from WITCO),capric and caprylic acid triglycerides (“MIGLYOL 812” from HULS).

The compositions may also contain thickeners which may be selected fromcross-linked or non cross-linked acrylic acid polymers, and particularlypolyacrylic acids which are cross-linked using a polyfunctional agent,such as the products sold under the name “CARBOPOL” by the companyGOODRICH, cellulose, derivatives such as methylcellulose,hydroxymethylcellulose, hydroxypropyl methylcellulose, sodium salts ofcarboxymethyl cellulose, or mixtures of cetylstearyl alcohol andoxyethylenated cetylstearyl alcohol containing 33 moles of ethyleneoxide.

When the compositions of the present invention are sunscreens they maybe in a form of suspensions or dispersions in solvents or fattysubstances or as emulsions such as creams or milks, in the form ofointments, gels, solid sticks or aerosol foams. The emulsions mayfurther contain anionic, nonionic, cationic or amphoteric surface-activeagents. They may also be provided in the form of vesicular dispersionsof ionic or nonionic amphiphilic lipids prepared according to knownprocesses.

In another aspect the present invention provides a method for preparingthe compositions of the present invention which comprises associatingthe particles described above with a carrier.

In another aspect the present invention provides particles comprising ahost lattice incorporating a second component to provide luminescencetrap sites and/or killer sites.

In a further aspect the present invention provides particles whichcomprise a population of coated nanoparticles of a metal oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the absorption of a photon of UV light by titanium dioxideas found in conventional sunscreens.

FIG. 2 shows the absorption of a photon of UV light by reduced zincoxide particles.

FIG. 3 shows the absorption of UV light by particles of the presentinvention which comprise at least two coated nanoparticles of titaniumdioxide.

FIG. 4 shows relaxation of plasmids caused by illuminated TiO₂ and ZnOand suppression by DMSO and mannitol. In both panels, S, L and R showthe migration of supercoiled, linear and relaxed plasmid. The top panelshows the plasmid relaxation found after illumination with sunlightalone for 0, 20, 40 and 60 min (lanes 1-4) and with 1% anatase (lanes5-8) or 1% rutile (lanes 9-12 ) TiO₂ for the same times. Lanes 13-18shows illumination with TiO₂ from sunscreen SN8 for 0, 5, 10, 20, 40 and60 min. The results are typical of those found with various samples. Thebottom panel shows illumination with 0.2% ZnO for 0, 10, 20, 40 and 60min before (lanes 1-5) or after (lanes 6-10) adding DMSO; and with0.0125% sunscreen TiO₂ for 0, 5, 10, 20, 40 and 60 min after adding 200mM DMSO (lanes 11-16) or 340 mM mannitol (lanes 1-22).

FIG. 5 shows the effect of catalase on damage inflicted by illuminatedTiO2 and location of lesions in DNA. The top panel shows plasmid DNAwhich was illuminated (see FIG. 4) with sunscreen TiO₂ alone for 0, 20,40 and 60 min (lanes 1-4) and for the same times (lanes 8-11) afteradding 2.5 units/μl of catalase (0.1mg/ml of protein). Lanes 5-7 showsupercoiled, linear and relaxed plasmid. The bottom panel showsillumination with sunscreen TiO₂ as above after adding boiled catalase(lanes 1-4) or 0.1 mg/ml of bovine serum albumin (lanes 8-11). The rightpanel shows a 426 bp fragment of double-stranded DNA labelled at one5′-end which was illuminated in 0.0125% sunscreen TiO₂ and samples whichwere analyzed on a sequencing gel. Lanes 1-4 show illumination for 0,20, 40 and 60 min. Lanes 5-8 show illumination for the same timesfollowed by treatment with N,N′-dimethylethylenediamine for 30 min at90° C. before analysis. This reagent displaces many damaged residuesfrom DNA and then cleaves the sugar-phosphate chain, leavinghomogeneous, phosphorylated termini with consistent mobility, thusclarifying the spectrum of lesions generated. Lanes 9-10 show G and Adideoxy sequencing standards.

FIG. 6 shows the damage inflicted on human cells revealed by cometassays. Row A shows comets obtained using X-rays from a Gavitron RX30source. The dose rate was 8.9 Gy min³¹ ¹ and cells were exposed on icefor 0, 15, 30 and 60 s, giving comets falling into the five mainstandard classes shown. 1, class 0; 2, class 1; 3, class II; 4, classIII; 5, class IV. Rows B and C show examples of comets obtained usingsimulated sunlight, MRC-5 fibroblasts and sunscreen TiO₂ (0.0125%). Foreach exposure, 100 cells were scored, and comets were classified bycomparison with the standards (row A). Row B shows no treatment (1);sunlight alone for 20, 40 and 60 min (2-4); and effect of TiO₂ in thedark for 60 min (5). Row C shows sunlight with TiO₂ for 0, 20, 40 and 60min (1-4); and for 60 min with TiO₂ and 200 mM DMSO (5). The chartssummarise results from five independent experiments. D shows thatsunlight alone inflicts few strand breaks and/or alkali-labile sites andE that inclusion of TiO₂ catalyses this damage.

FIG. 7 shows a comparison of strand breaks inflicted on DNA by sunlightin the presence of either normal zinc oxide or the zinc oxide of thepresent invention. Lanes 1-4 show illumination with light alone for 0,20, 40, 60 minutes; lanes 5-8 show illumination with normal ZnO(Aldrich) for 0, 10, 20, 40 and 60 minutes; and lanes 9-12 showillumination with reduced ZnO for 0, 20, 40 and 60 minutes.

FIG. 8 shows the degradation of DNA irradiated in the presence ofmanganese doped titanium dioxide. The Figure shows the number of strandbreaks per plasmid found during illumination with simulated sunlight ata total intensity between 290 and 400 nm of 6 mWatts/cm². Light aloneinflicted significant damage (circles). With anatase titanium dioxidethe damage was so severe that it could not be accurately quantified.With rutile titanium dioxide the damage was less (squares), but stillsevere enough to run off the scale at early times. The presence ofmanganese reduced this damage considerably. Both 0.1% (triangles) and0.5% (crosses) manganese had very similar effects, reducing the rate ofinflicting damage by about 70%, but an increase to 1% (diamonds) had avery beneficial effect, reducing the damage to undetectable levels atthe light doses used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Examples which follow farther illustrate the present invention withreference to the figures.

COMPARATIVE EXAMPLE

Chemical Oxidation by Titanium Dioxide Preparations TiO₂ samples wereextracted from over-the-counter sunscreens by washing with organicsolvents (methyl cyanide, acetone, chloroform), and their anatase andrutile contents were determined by X-ray diffraction methods. Anataseand rutile standards were obtained from Tioxide Group Services Ltd.,Grimsby, UK. TiO₂ concentrations were assayed according to the method ofCodell [M. Codell, (1959), Analytical Chemistry of Titanium Metal andCompounds, Interscience, New York] using standards made from pure TiO₂(Aldrich); the molar extinction coefficient for the complex was assayedas 827 M⁻¹ at 404 nm. The photo-oxidation degradation of phenol byilluminated TiO₂ was monitored using high pressure liquid chromatography[N. Serpone et al., (1996), J. Photochem. Photobiol. A: Chem 94,191-203] to measure its disappearance, employing isocratic procedures atambient temperature on a Waters 501 liquid chromatograph equipped with aWaters 441 detector set at 214 nm and a HP 3396A recorder. The columnwas a Waters μBONDAPAK C-18 reverse phase and the mobile phase was a50:50 mixture of methanol (BDH Omnisolv grade) and distilled/deionisedwater. Each sunscreen TiO₂ was illuminated at 0.05% by weight in 58 mlof phenol (200μM in air-equilibrated aqueous media, Ph 5.5; retentiontime of phenol in the HPLC chromatogram was 5 min) using a 1000-W Hg/Xelamp and a 365 nm (±10 nm) interference filter, giving a light fluxbetween 310 and 400 nm of ca. 32 mW cm^(2−.)

Appropriate aliquots (1ml) of the irradiated dispersion were taken atvarious intervals and filtered through a 0.1 μm membrane to remove theTiO₂ prior to analysis.

Illumination of DNA in Vitro The solar simulator [J. Knowland et al.,(1993), FEBS Lett. 324, 309-313] consists of a 250-W ozone-free lamp, aWG 320 filter and a quartz lens, resulting in an estimated fluencebetween 300 and 400 nm of 12 W m⁻² DNA was the plasmid pBluescript IISK⁺ (Stratagene) prepared and analyzed on agarose gels according to [T.Maniatis et al., (1982), Molecular Cloning: A Laboratory Manual, ColdSpring Harbor Laboratory, Cold Spring Harbor, N.Y.]. Relaxed standardswere made by depurinating plasmid in 25 mM sodium acetate pH 4.8 at 70°C. for 20 min followed by cleaving with exonuclease III at 37° C. [N.Serpone, (1997), Photochem. Photobiol. A. 104, 1] in 50 mM Tris-HCl, 5mM CaCl₂ (the Ca²⁺ inhibits exonuclease but not cleavage at apurinicsites), 0.2 mM DTT, pH 8; linear standards by cutting with EcoRI. Theauthentic TiO₂ standards (confirmed by X-ray diffraction to be 100%anatase or 100% rutile) were suspended in water at 2% w/v; ZnO (Aldrich,<1 μm) at 0.4% w/v. 25 μl of each were added to 25 μl of plasmid (2-3 μgof DNA) in 100 mM sodium phosphate pH 7.4 and illuminated as droplets(50 μl) on siliconised microscope slides placed on a brass blockembedded in ice. A sunscreen containing only TiO₂ (7% w/v) was vortexedwith water and centrifuged. The white pellet was washed 3 times with amixture of chloroform and methanol (1:1), then with methanol alone, anddried. The powder was suspended in water at 2%, but most quickly settledout, leaving a cloudy supernatant with a TiO₂ content assayed at 0.025%w/v. This was mixed with an equal volume of plasmid DNA in buffer andilluminated. Direct strand breaks were assayed from the conversion ofsupercoiled plasmid to the relaxed form.

Illumination of DNA in Vivo (Comet Assays)

Human cells (MRC-5 fibroblasts) were illuminated on ice with or withoutsunscreen TiO₂ (0.0125% w/v). The lens was omitted, giving an intensitysimilar to that found under the stratum corneum [Knowland, J. et al.,(1993), FEBS Lett. 324, 309-313]. Samples were taken at increasingtimes, kept on ice, and analysed at the same time. For analysis, cellswere embedded in low-melting agarose, lysed with 1% Triton X-100,subjected to alkaline gel electrophoresis and stained with ethidiumbromide [P. W. Doetsch and R. P. Cunningham, (1990), Mutat. Res. 13,3285-3304], and classified according to the five main standard classes[V. J. McKelvey-Martin et al., (1993), Mutat. Res. 228, 47-63].

Oxidation of organic materials by hydroxyl radicals from illuminated TiOcan be examined conveniently by following the oxidation of a testmolecule such as phenol [N. Serpone et al., (1996), J. Photochem.Photobiol. A: Chem. 94, 191-203]. The table below compares the oxidativedegradation of phenol by TiO₂ samples from 10 different sunscreens withoxidation catalysed by pure rutile and pure anatase.

TABLE Photodegradation of phenol by TiO₂ samples Anatase/rutile Phenolphotodegradation Relative Sample ratio (%) (mmol h⁻¹) rate SN1^(a)50/50  0.008 ± 0.016 1.0 SN2 0/100 0.023 ± 0.008 2.8 SN3 0/100 0.043 ±0.010 5.2 SN4 54/46  0.043 ± 0.007 5.2 SN5 0/100 0.086 ± 0.015 10.4 SN6100/0   0.146 ± 0.014 17.6 SN7^(a) 0/100 0.189 ± 0.008 22.7 SN8 100/0  0.44 ± 0.11 53.3 SN9 63/37  1.11 ± 0.03 134 SN10^(b) 0/100 1.50 ± 0.04180 Pure rutile 0/100 3.55 ± 0.12 427 Pure anatase 100/0   31.6 ± 0.8 3803 SN1-10 are over-the-counter sunscreens. ^(a)Also contains Al(OH)₃.^(b)Also contains 1.95% ZnO.

All TiO₂ samples oxidise phenol, but activity does not depend solely oncrystal type. The most active sample, SN10, also contains ZnO.

Hydroxyl radicals inflict direct strand breaks on DNA, and to test forsuch damage supercoiled plasmids were illuminated with simulatedsunlight and TiO_(2.) FIG. 4 shows that plasmids were converted first tothe relaxed form and ultimately to the linear form, demonstrating strandbreakage. Sunlight alone had very little effect, while anatase is moreactive than rutile, consistent with photochemical comparisons (Table 1and [A. Sclafani and J. M. Herrmann, (1996), J. Phys. Chem. 100,13655-13661]. TiO₂ extracted from a sunscreen is also photo-active, andso is pure ZnO. The sunscreen illuminations contain much less TiO₂ thanthe anatase and rutile ones, suggesting that the sunscreen variety isespecially active. Damage was suppressed by the quenchersdimethylsuphoxide (DMSO) and mannitol, suggesting that it is indeedcaused by hydroxyl radicals.

FIG. 5 shows (top panel) that damage was very slightly suppressed bycatalase, but also (bottom panel) that heat-inactivated catalase andbovine serum albumin have similar effects, suggesting that this limitedquenching was due to the protein present rather than to catalaseactivity. Superoxide dismutase did not suppress the damage either (datanot shown). It appears therefore that the strand breaks are not causedby superoxide (O₂ ^(.−)), an active oxygen species formed by reactionbetween e³¹ (cb) and O₂, and do not depend upon the intermediateformation of hydrogen peroxide by reaction between 2^(.)OH radicals.Rather, they appear to be due to direct attack by hydroxyl radicals,which is consistent with indications that hydroxyl radicals formed onTiO₂ remain on the surface of the particles. By cleaving end-labelledDNA, other lesions were revealed (right panel), principally at some, butnot all, guanine residues. Evidently, DNA damage is not confined tostrand breaks.

Comet assays (FIG. 6) show that DNA in human cells is also damaged byilluminated TiO₂, consistent with endocytosis of TiO₂. Suppression byDMSO again implies that the damage is caused by hydroxyl radicals. Theseassays detect direct strand breaks and alkali-labile sites, and revealthe damage attribution to TiO_(2.)

Thus, illuminated sunscreen TiO₂ and ZnO can cause oxidative damage toDNA in vitro and in cultured human fibroblasts. This has importantimplications for use of conventional compositions for topicalapplications. Autoradiographic studies using ⁶⁵ZnO have suggested thatit passes through rat and rabbit skin, probably through hair follicles,although the chemical form of the ⁶⁵Zn detected under the skin (andhence of the form that crosses the skin) is not clear. Some reports haveraised the possibility that ZnO and pigmentary TiO₂ pass through humanskin, and a recent one suggests that micronised TiO₂ in sunscreens doestoo [M.-H. Tan et al, Austalas. J. Dermatol, (1996), 37, 185-187]. It istherefore important to characterise the fate and photochemical behaviourof sunscreens, which certainly prevent sunburn, because they are alsointended to reduce skin cancers, which have increased rapidly recently.

EXAMPLE 1

Preparation of Reduced Zinc Oxide Particles

The zinc oxide used was supplied by Aldrich. It has a particle size of100-200 nm and absorbs in the UV below 390 nm and reemits at 390 nm. Itwas heated in an atmosphere of 10% hydrogen to 90% nitrogen by volume at800° C. for 20 minutes. Reduced zinc oxide was obtained and was found toabsorb in the UV and reemit in the green at 500 nm.

EXAMPLE 2

Comparison of Strand Breaks Inflicted on DNA by Sunlight in the Presenceof Either Normal or Reduced Zinc Oxide.

The DNA used was the plasmid pBluescript II SK⁺ (Stratagene) preparedand analyzed on agarose gels according to T. Maniatis et al. [MolecularCloning: A Laboratory Manual. Cold Spring Harbor Laboratory, Cold SpringHarbor, N.Y. (1982)]. It was illuminated in vitro using a solarsimulator [J. Knowland et al, FEBS Lett., 324 (1993) 309-313] consistingof a 250 watt ozone-free lamp, a WG 320 filter and a quartz lens,resulting in an estimated fluence between 300 and 400 nm or 12Watts.m^(. 2). Zinc oxide samples were suspended in water at 0.4% w/v.25 μl of each suspension was added to 25 μl of plasmid (2-3 μg of DNA)in 100 mM sodium phosphate pH 7.4 and illuminated as droplets (50μl) onsiliconised microscope slides placed on a brass block embedded in ice.Direct strand breaks were assayed from the conversion of supercoiledplasmid to the relaxed form, marked S and R on FIG. 7.

It was observed that illuminated reduced ZnO particles are very muchless liable to produce DNA damage than conventional ZnO particles.

EXAMPLE 3

Preparation of Manganese Doped Titanium Dioxide.

Titanium dioxide (25 g) and manganese (II) nitrate (0.8 g) were mixed indeionized water (100 ml). The resulting mixture was ultrasonicated for10 minutes and then boiled dry. The material produced was fired at 700°C. for 3 hours to give 1% manganese doped titanium dioxide. Titaniumdioxide particles with differing dopant levels were prepared in ananalogous manner by varying the amount of manganese (II) nitrate.

It is believed that manganese 3+ions (oxidised from manganese 2+duringboiling) are incorporated into the surface of the titanium dioxide hostlattice.

EXAMPLE 4

Assessment of the Effect of Varying the Percentage of Manganese in aTitanium Dioxide Host Lattice.

The ability of different manganese doped titanium dioxide particles todamage DNA was measured using the plasma nicking assays described in theComparative Example above.

FIG. 8 shows that simulated sunlight alone inflicted significant damageon DNA as revealed by the generation of strand breaks. As the percentageof manganese in the titanium dioxide host lattice was increased thedamage in terms of strand breaks decreased. At a manganese content of 1%the damage was almost prevented and was significantly less than thedamage inflicted by light alone.

Thus it is believed that such particles may protect DNA from thisparticular form of damage. The particles may absorb photons of lightwhich would normally inflict damage and divert their energy intonon-damaging processes. The particles may work by photoexcited carriertrapping or by the dopant ions rendering the host lattice p-type. It isbelieved that these mechanisms may be connected. For particles of lessthan 20 nm the quantum size effect may also help by shifting the redoxpotentials.

1. A UV screening composition comprising particles having an averageprimary particle size not exceeding 100 nm which are capable ofabsorbing UV light so that electrons and positively charged holes areformed within the particles, characterised in that the particles areadapted to minimise migration to the surface of the particles of theelectrons and/or the positively charged holes when said particles areexposed to UV light in an aqueous environment said particles being zincoxide particles incorporating manganese ions or zinc oxide particlesincorporating chromium ions, and a carrier.
 2. A composition accordingto claim 1 wherein the chromium or manganese ions are in the 3+ state.3. A composition according to claim 2 wherein the chromium or manganeseions are present in an amount of from about 0.1 to 1 atom %.
 4. Acomposition according to claim 1 wherein the size of the particles isfrom 1 to 50 nm.
 5. A composition according to claim 1 wherein theparticles have an outer coating.
 6. A method for preparing a compositionas defined in claim 1 which comprises associating the particles with acarrier.
 7. A composition according to claim 1 wherein said compositionis formulated for cosmetic topical application.
 8. A compositionaccording to claim 7 wherein said composition is a sunscreen.
 9. Acomposition according to claim 1 formulated as a paint or coating. 10.The UV screening composition of claim 1, further comprising UVA, UVB orbroad-band sunscreen agents.